The relationships between HbA1c level and oral glucose tolerance test (OGTT) at the initial visit and the incidence of diabetes after 5 years of follow-up were investigated in 819 subjects participating in a general health examination. The 100 g OGTT was performed. In order to use WHO criteria, the blood glucose levels of 100 g OGTT corresponding to those of 75 g OGTT were adopted according to the recommendations of the Japan Diabetes Society. Subjects other than diabetic type and IGT (impaired glucose tolerance) were divided into a normal group (fasting blood glucose < 100 mg/dl, 1-h blood glucose < 160 mg/dl, a 2-h blood glucose < 120 mg/dl) and a borderline group (the remaining subjects). In IGT, the incidence of diabetes in the low- (< or = 6.3%), intermediate- (6.4-6.7%) and high-HbA1c (> of = 6.8%) groups were 10.4%, 23.1% and 52.5%, respectively (high vs intermediate and low, P < 0.001; intermediate vs low, P < 0.05). In the borderline group, the incidence were 2.8%, 14.3% and 28.6%, respectively (high and intermediate vs low, P < 0.001). The results showed that the combination of HbA1c level and OGTT enables more precise prediction of progression to NIDDM in subjects with glucose intolerance.